This paper has been submitted at Nov 4th, 1996, for publication in a scientific journal and is now subject to the reviewing process.
You may ask for a hardcopy with the complete manuscript, which contains all figures and all greek symbols.
Everybody is invited to comment to this paper. I will consider every argument and comment back.
This is the 3rd revised version.

Dr. Bernhard Wessling
Zipperling Kessler / Ormecon Chemie
D-22949 Ammersbek

Conductive Polymer / Solvent Systems: Solutions or Dispersions?

1.2 Origin of surface properties

What is the reasoning behind this agreement to have a specific term (and a whole science) for this class of mixtures? Let's find out what we are changing when starting with a bulk solid (e. g., a sphere with a diameter of d = 1 cm) and transform it stepwise into a colloidal dispersion and then - with further going to the molecular scale - into a solution. It is obvious, that we are first creating new surfaces with every subdivision step.

The surface area (and hence the interfacial area between the dispersed phase and the dispersing medium) is given in [4a] by eqn (1):

(1) Area / Mass = 6 / d

For a material with the density = 2 g/cm3 the specific area develops like that:

 diameter     area/g    
      1 cm     3 cm2  
      1 mm    30 cm2  
 10 micron    0,3 m2  
  1 micron      3 m2  
    100 nm     30 m2  
     10 nm    300 m2  
      1 nm   3000 m2

When going further to the molecular dispersion (= solution), we will suddenly have no interfacial area at all any more, as we are dealing then with a homogeneous system (and single molecules do not have a "surface" in the sense of bulk or particulate liquids or solids). In dispersion, all particle surfaces are forming an interface with the dispersion medium.

We now can conclude: Colloidal dispersions are the only class of mixtures of different chemical species where the interfacial area plays a dominant or at least an important role, so that their properties are significantly impacted or even mainly determined by interfacial forces or interactions. Colloids are not a special class of chemicals, but chemical species in a very special state.

Neither a bulk chemical or a macroscopically heterogeneous mixture (like a suspension of stones in a river or a mixture of lottery balls) nor a molecular dispersion (solution) of a species in a solvent are exhibiting such a partially huge specific surface area as we find for colloids. It is common to have an interfacial area of 10 to 100 m2/ml dispersion, the living area in a student's room or in a scientist's house folded into one single milliliter!

The phenomenon, that the interfaces are playing such a dominant role comes from the fact, that molecules situated at the surface are different (and behave different) from those in the bulk. This is easy to understand when looking at a sodium chloride (Na+Cl-) crystal. Every Na+ "tries" to have 6 Cl- around itself, and vice versa, leading to the well-known octahedral crystal structure. But what is the situation at the surface? Here, a given Na+ is only surrounded by 5 Cl-, and there is a partially "naked" Na+. This will lead to either some distortion and irregularity in the crystal structure only at the surface, or to an adsorption of, e. g. water for shielding the charge.

The same is true for every other kind of solid, be it an inorganic or an organic compound. For instance in the case of diamond, the carbon atoms at the surface have unsaturated bonds towards the outside. But also molecules, even unpolar ones, attract each other to build liquids or solids. The attractive forces between non-polar molecules have been studied by van der Waals and theoretically interpreted by London. The size and form of the London forces, depending on the polarizability and the first ionization potential, is responsible for the cohesive forces between identical molecules, which in the case of solids lead to the crystal (or glassy) structure.

The formation of salts and their structure is due to additional factors discussed later.

For the surface molecules, there are no molecules available identical with those in the bulk, which they could attract from outside, so that their attraction forces are partially directed to the outside, either into the vacuum, or into a gaseous atmosphere, or into a liquid dispersion medium. This leads to distortions of the particle structure near the surface, and to complex interactions with molecules of the medium at the surface. It also results in a chemical composition of the surfaces often significantly different from the bulk, see fig. 1, 2 and 3.

As long as the surface area is negligible in relation to the bulk, we do not find specific properties arising from interfacial interactions. This is the case if only a negligible number of molecules is to be found at the surface in comparison to the number in the bulk. Silver bromide is a substance with important technical application in photographic films. It is used in a colloidal dispersion. An AgBr cube of 1 cm side length contains only about 2 ions out of ten million at the surface. When divided into 1 µ particles, we find 1 ion out of 450 at the surface. At 10 nm size, 25% of the ions are at the surface!

Specific colloidal properties (like rheological, light scattering and others) can be found in the range where surface molecules begin to play an important role (1 µ) or even dominate the properties of the system (10 - 1 nm). Above 1 µ the surface area is negligible, below this, there is no surface any more.

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